Experts from the World Health Organization have all but eliminated a controversial theory that Covid-19 came from a laboratory in the Chinese city of Wuhan.

"The laboratory incident hypothesis is extremely unlikely to explain the introduction of the virus into the human population," said Dr Peter Ben Embarek, the head of the WHO mission.

"Therefore is not in the hypotheses that we will suggest for future studies."

He said the team's investigation had uncovered new information but had not dramatically changed the picture of the outbreak.

Dr Ben Embarek also told a press briefing that work to identify the origins of the coronavirus points to a natural reservoir in bats, but it is unlikely that they were in Wuhan.

Earlier, the head of the China team said studies showed the virus "can be carried long-distance on cold chain products," appearing to nudge towards the possible importation of the virus - a theory that has abounded in China in recent months.

Liang Wannian also said there was "no indication" the sickness was in circulation in Wuhan before December 2019 when the first official cases have been recorded.

Dr Ben Embarak, who was based in the WHO's Beijing office for two years from 2009, backed up the assertion saying there was no evidence of "large outbreaks in Wuhan" before then.

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During the closely monitored visit, reporters were largely kept at arm's length from the experts, but snippets of their findings crept out over Twitter and interviews.

The experts spent one month in China, two weeks in quarantine and the same again on fieldwork.

But, already over a year after the virus emerged, some of it was of questionable relevance to their stated aim of finding the virus source, including a visit to a propaganda exhibition celebrating China's recovery from the pandemic.

The group spent just an hour at the seafood market where many of the first reported clusters of infections emerged over a year ago.

They also appeared to spend several days inside their hotel, receiving visits from various Chinese officials without going out into the city.

But deeper research was carried out at the Wuhan virology institute where they spent nearly four hours and said they met Chinese scientists there, including Shi Zhengli, one of China's leading experts on bat coronaviruses and deputy director of the Wuhan lab.

Former US president Donald Trump repeated a controversial theory that a lab leak may have been the source of the pandemic.

Scientists at the laboratory conduct research on some of the world's most dangerous diseases, including strains of bat coronaviruses similar to Covid-19.

Beijing is desperate to diminish criticism of its handling of the chaotic early stages of the outbreak.

It has refocused attention at home and abroad on its handling of, and recovery from, the outbreak.